Biochar colloids mobilization by consecutive fluid displacement in unsaturated condition

The theoretical narratives of transport and movement of biochar colloids (BCs) exist mostly for saturated zones or groundwater in the literature, while their corresponding mobilization mechanisms during drainage and imbibition in unsaturated systems have not been deliberated in detail. The main objective of this study is to quantify the BCs attached to the sand surfaces after passing the consecutive sequential fluid displacement cycles. The joint effect of change in flow rates and salinity of the solution was estimated in the artificially created unsaturated condition due to moving air-water interfaces over the BC deposited sediment. Initially, the synthesized BCs were deposited over silica sand. Then three flow rates (0.2, 1.0, 5.0 mL/min) and two salinity strengths (0.2 mM and 8 mM NaCl) were used to perform a series of unsaturated column transport experiments. The BC was also characterized by its elemental composition, functional groups, and surface morphologies. The results indicated that the BC could mobilize during the first sequential fluid displacement cycle while the salinity of the background solution was low. However, in a subsequent fluid displacement cycle, the BC was released while the salinity of the background solution was high. This phenomenon likely happened due to the attachment force between BCs and sand surfaces, as the primary energy barrier could be much stronger for higher salinity conditions. It is concluded that relatively smaller BCs could be detached from the sediment irrespective of flow rates or salinity strength used during the fluid displacement cycle. Based on these results, it can be inferred that BC present in the agricultural field effectively preserves the nutrients and supplies as a slow-release fertilizer in the event of intermittent rain during the monsoon season.